1. Field of the Invention
This invention relates in general to magnetic transducers for reading information signals from a magnetic medium and, in particular, to an improved magnetoresistive read transducer.
2. Description of the Prior Art
The prior art discloses a magnetic transducer referred to as a magnetoresistive (MR) sensor or head which has been shown to be capable of reading data from a magnetic surface at great linear densities. An MR sensor detects magnetic field signals through the resistance changes of a read element made from a magnetic material as a function of the amount and direction of magnetic flux being sensed by the element. These prior art MR sensors operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the resistance varies as cos.sup.2 of the angle between the magnetization and the direction of current flow. These MR sensors have operated on the basis of the AMR effect even though this effect produced only a very small percentage change in the resistance.
More recently, reports have been published of techniques for obtaining enhanced MR effects. One of these publications, "Enhanced Magnetoresistance in Layered Magnetic Structures with Antiferromagnetic Interlayer Exchange", G. Binasch, et al, Physical Review B. V39, p. 4828 (1989) describes a layered magnetic structure which yields enhanced MR effects caused by antiparallel alignment of the magnetization. However the saturation fields required to obtain these changes in resistance were too high and the effect too nonlinear and therefore unsuitable for producing a practical MR sensor.
Another of these publications, "Layered Magnetic Structures: Exchange Coupling of Ferromagnetic Layers Across Interlayers", P. Grunberg et al., MRS Intl. Mtg. Adv. Mats., V. 10, p. 255 (1989), and studies dating back to the 1960's have shown ferromagnetic coupling in such systems decaying monotonically with increasing separation of the magnetic layers for a wide range of systems.
The prior art has not shown such a multilayered system in which the magnetoresistance is high and in which the fields are sufficiently low so that it is useful as an MR sensor.